VNA Coupler Emulation Circuit for DC S-Parameter Measurement
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Solution Overview
Problem
Conventional vector network analyzers (VNAs) are limited by the low frequency range of their couplers, typically not able to measure below 1 or 2 MHz due to the balun and Ferrite beads used, making it impractical to measure S-parameters down to DC frequencies.
Innovation Solution
A circuit is introduced in the signal path of the VNA, comprising a pi-attenuator and a dual directional coupler emulation circuit made of op-amps and resistors, allowing the VNA to emulate the behavior of a dual directional coupler at frequencies below the actual coupler's limit, enabling measurements down to DC using a pi-attenuator and op-amp-based coupler emulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional couplers with Ferrite beads are used, then the coupler structure is simple and easy to manufacture, but the measurement frequency range is limited to above 1-2 MHz
Solution Approach 1:
The measurement system is segmented into two frequency ranges: DC to low frequency (handled by emulation circuit) and high frequency (handled by conventional coupler). This segmentation allows each part to operate within its optimal range, extending overall measurement capability while maintaining manufacturing simplicity for the high-frequency portion.
Solution Approach 2:
A dual directional coupler emulation circuit acts as an intermediary for low-frequency measurements, replacing the physical coupler in this frequency range. The emulation circuit uses op-amps and resistors to simulate coupler behavior, enabling DC to low-frequency measurements without modifying the conventional high-frequency coupler structure.
2Adaptability or versatility
If multiple Ferrite beads are added to extend low frequency range, then the measurement frequency range decreases, but the device complexity and cost increase prohibitively
Solution Approach 1:
The physical mechanical structure of multiple Ferrite beads is replaced with an electronic emulation circuit using op-amps and resistors. This substitution achieves the same low-frequency measurement capability without the physical complexity and cost of stacking multiple beads, as the emulation circuit electronically simulates the required coupling behavior.
3Adaptability or versatility
If switches are used to switch between different measurement modes, then the operational flexibility improves, but switching uncertainties and additional losses are introduced
Solution Approach 1:
The system dynamically adapts to the input signal frequency by having the emulation circuit naturally take over at low frequencies and the conventional coupler operate at high frequencies. This dynamic behavior is achieved through frequency-dependent circuit characteristics rather than active switching, eliminating switching uncertainties and maintaining measurement reliability across the entire frequency range.
Data Source
AI summary
Embodiments of the present invention relate to circuits to be inserted in a signal path between a signal generator and a test port of a vector network analyzer (VNA), where the circuits enable the VNA to make scattering parameter measurements for a load (RL) connected to the test port when the signal generator generates signals having frequencies that are below a low frequency limit (e.g., 2 MHz) of an actual dual directional coupler of the VNA. Embodiments of the present invention are also directed to a VNA that includes such circuits.